In recent years, large-sized and high-resolution televisions are becoming more and more popular among merchants and customers. As size of a display device becomes larger and larger, on the premise of ensuring a starting current (Ion) and a threshold voltage (Vth), electric-capacity-holding effects of a display device is becoming more and more important. Since a liquid crystal display device is in an off-state during most of the time, a leakage current exerts a great influence on display performance of the device.
Generally, for a BCE (B stands for base, C for collector and E for emitter) transistor structure, after a channel is formed, there are many follow-up procedures like depositing metal. The follow-up procedures will influence characteristics of a backchannel. An active layer usually adopts an AL/AH structure. AL is a main electrically-conductive channel, and is formed at a low speed, so as to ensure good thin-film quality and few interfacial defects. AH, formed at a high speed, has many interfacial defects, and many interfacial states in a backchannel. While a gate voltage Vg is a small negative voltage, in other words, while a transistor is in an electrically-conductive negative sub-threshold region, a small electrically-conductive channel will be formed in the backchannel, which will lead to a backchannel electric leakage. In the meanwhile, a turn-off voltage of a TFT-LCD device is generally provided around the negative sub-threshold region. Besides, due to poor homogeneity and many defects of the deposition, AH is easily affected by following procedures, resulting in a high and dispersed leakage current in the backchannel and leading to problems like uneven luminance display, reliability problems, etc.
Thus, to improve performance of a display device, it is of great significance to improve homogeneity of a backchannel and interfacial characteristics of the active layer in the backchannel. Performance of the sub-threshold region I-V of the device, as shown in FIG. 1, is mainly affected by the backchannel of a device and interfacial characteristics of the backchannel and a passivation layer PV.